Seismic surveys are conducted to map subsurface structures to identify and develop oil and gas reservoirs. Seismic surveys are typically performed to estimate the location and quantities of oil and gas fields prior to developing the fields (drilling wells) and also to determine the changes in the reservoir over time subsequent to the drilling of wells. On land, seismic surveys are conducted by deploying an array of seismic sensors (also referred to as seismic receivers) over selected geographical regions. These arrays typically cover 75-125 square kilometers or more of a geographic area and include 2000 to 5000 seismic sensors. The seismic sensors (such as, geophones or accelerometers) are coupled to the ground in the form of a grid. An energy source, such as an explosive charge (buried dynamite for example) or a mobile vibratory source, is used at selected spaced apart locations in the geographical area to generate or induce acoustic waves or signals (also referred to as acoustic energy) into the subsurface. The acoustic waves generated into the subsurface reflect back to the surface from subsurface formation discontinuities, such as those formed by oil and gas reservoirs. The reflections are sensed or detected at the surface by the seismic sensors. Data acquisition units deployed in the field proximate the seismic sensors may be configured to receive signals from their associated seismic sensors, at least partially process the received signals, and transmit the processed signals to a remote unit (typically a central control or computer unit placed on a mobile unit). The central unit typically controls at least some of the operations of the data acquisition units and may process the seismic data received from all of the data acquisition units and/or record the processed data on data storage devices for further processing. The sensing, processing and recording of the seismic waves is referred to as seismic data acquisition.
The traditional sensor used for acquiring seismic data is a geophone. Multi-component (three-axis) accelerometers, however, are more commonly used for obtaining three-dimensional seismic maps. Compared to seismic surveying layouts using the single component sensors, layouts using multi-component sensors require use of more complex data acquisition and recording equipment in the field and a substantially greater bandwidth for the transmission of data to a central location.
A common architecture of seismic data acquisition systems is a point-to-point cable connection of all of the seismic sensors. Typically, output signals from the sensors in the array are collected by data acquisition units attached to one or more sensors, digitized and relayed down the cable lines to a high-speed backbone field processing device or field box. The high-speed backbone is typically connected via a point-to-point relay with other field boxes to a central recording system, where all of the data are recorded onto a storage medium, such as a magnetic tape.
Seismic data may be recorded at the field boxes for later retrieval, and in some cases a leading field box is used to communicate command and control information with the central recording system over a radio link (radio frequency link or an “RF” link). Even with the use of such an RF link, kilometers of cabling among the sensors and the various field boxes may be required. Such a cable-system architecture can result in more than 150 kilometers of cable deployed over the survey area. The deployment of several kilometers of cable over varying terrain requires significant equipment and labor, often in environmentally sensitive areas.
Due to the high speed of acoustic waves through earth formations and the sensitivity required to resolve seismic signals, timing precision is of considerable importance in seismic data acquisition. Traditionally, timing precision has been achieved through clock synchronization of remote units arranged in a point-to-point topology with a master clock. However, these systems are limited in configuration since the master clock must interface with each of the remote units individually. What is needed is a system that allows for clock synchronization to be implemented over remote units arranged using a line and/or tree topology. This disclosure discusses such a system.